The present invention relates to a non-slip type rectilinear wiredrawing machine with tangentially uncoiling capstans incorporating a synchronization device between each two successive capstans.
Conventionally, in a multiple drawing machine for the manufacture of metal wire, where each drawing step reduces the diameter of the wire by a given percentage of its rounded section, the fundamental difficulty encountered is that of synchronizing the rotational speeds of the capstans, which in essence function as collect-and-feed stations intercalated with the successive drawing dies or plates in such a way as to ensure a steady flow of material. Thus, expressing the velocity and section of the wire per drawing step (n) as Vn and Sn, it must be ensured that Sn.times.Vn=k.
The product of section multiplied by speed, i.e. the volume of the flow of material, must in effect remain constant from one step to the next. Given therefore that the section of the wire is dependent on the diameter of the drawing die or plate located between capstans, and that this same diameter will be subject to an unpredictable and uncontrollable degree of variation through wear during production, a correction can be effected only by varying the velocity of the wire which, in the non-slip type of drawing machine (i.e. where the capstan carries a significant number of single coils of wire, thereby disallowing relative movement between capstan and material), is equivalent to the peripheral surface speed of the capstans.
In multiple machines such as the Morgan and similar types, the wire is wound spirally onto cylindrical capstans and uncoiled in an axial direction from the capstan. Synchronization is achieved in such machines, necessarily, by operating the capstans intermittently, and while the flow of material is rendered steady in this manner, the result is but modestly successful. The main limitations of such machines stem from the need for intermittent type operation on the one hand, and on the other, from the fact that the wire is subjected to undesirable stresses; in effect, the wire is twisted through a full revolution with each coil paid out from the capstan, by reason of the axial uncoiling action. Moreover, these axially uncoiling machines require a device by means of which to transfer the running wire from one capstan to the next (an `uncoiler`, in effect), which comprises pulleys positioned one alongside and another elevated axially from the capstan, serving to direct the wire toward and into the drawing die preceding the next capstan.
In a variation on this type of machine, designed to prevent twisting of the wire (which is undesirable in any event, but absolutely to be avoided when drawing steel with a high carbon content), use is made of two capstans positioned one above the other with a single transfer pulley located in between that enables the wire to run off the second capstan tangentially instead of axially. The drawback of intermittent operation remains in such machines, however, in addition to the considerable structural complications that arise with two capstans to each drawing step.
With the advent of d.c. capstan drive motors, it has been possible to update these machines to newer technological standards; accordingly, the "stop/go" type of intermittent operation can be improved to "slow/fast", and by incorporating further special expedients and transducers, continuous and entirely intermittence-free operation can also be achieved. Also, the use of variable speed converters has led to the embodiment of new rectilinear wiredrawing machines in which the wire passes directly from one capstan to the next. The number of coils passing round each capstan remains fixed, and absolutely no twisting occurs in passage of the wire from step to step.
The capstans themselves are of frustoconical shape, exhibiting a gentle taper that enables and favors an orderly and substantially non-overlapping coil along the winding surface between the pulling face where the wire enters into full contact with the surface, and the run-out face at the very top of the capstan. Accordingly, the wire can be made to uncoil tangentially from such a capstan.
In the rectilinear machine, there is no slippage between the wire and the capstan face, so that the velocity of the wire coincides with the surface speed of the capstan. This automatically dictates the need to govern the tension of the wire between capstan; the necessary control is obtained in most instances by locating a jockey, or dancer, between one capstan and the next, and more exactly, between the exit of each capstan and the drawing die or plate next in sequence, positioned in such a way as to react to any geometrical variation in a loop of wire created between the two capstans for the very purpose in question. The dancer combines with a suitable transducer, of which the response varies with oscillation induced by changes in tension of the wire, to create a control medium of which the corresponding variation in output can be used to correct the speed of the interlocked capstan. In rectilinear machines of the type in question, the wire generally needs to be directed around one or more pulleys before entering the drawing die associated with the following capstan, in order to create a degree of slack sufficient to accommodate the excursion of the dancer; this results in a certain degree of drag on the loop of wire, of which the force will depend on the mechanical load applied to the dancer. Moreover, these pulleys are generally of diameter much smaller than that of the capstan, especially when installed in any number, so that the wire is subjected to a succession of alternate bending stresses; such an effect is not only undesirable, but especially damaging when the wire is still relatively thick during the initial drawing steps, or when operating with particularly large nominal production diameters. Conversely, if the dancer mechanism is reduced to a simple sensor monitoring a single loop of wire located between two capstans, the resulting control becomes so highly sensitive as to produce a critical operating characteristic, and flexibility is lost. Thus, notwithstanding the advantage of affording a speed control facility, even the rectilinear type of wiredrawing machine betrays not inconsiderable drawbacks.
Capstan speed can be governed by monitoring torque rather than speed, however, and this is the method adopted in a further type of machine in which speed is compensated by drag. The advantage of these machines consists in the fact that one has a direct transfer of the wire from one capstan to another, without dancers or other such devices; in practical terms, the wire passes directly from one capstan to the drawing die located between this and the next capstan. Synchronization is achieved automatically inasmuch as the drive of the interlocked capstan will not deliver the total required drawing torque, but a given proportion thereof, insufficient in any event to set the capstan in rotation. The remaining proportion is provided by the capstan next in line by way of the interconnecting wire, which generates the drag necessary to compensate the shortfall. The effect is passed on down to the final capstan in line, which, being speed-controlled, automatically determines the speed of all the preceding capstans. Whilst there are no problems with transfer of the wire from one capstan to the next in such machines, the compensating drag cannot be metered accurately to match the effective requirement, and the risk of the wire breaking is therefore greatly increased in consequence.
Furthermore, the matching of speeds between one capstan and the next is markedly rigid, given the absence of any margin of tolerance, or of any flow compensating means by which to take up the minute variations in velocity between capstans caused by an irregular flow of material.
Finally, optimum torque-metering of the capstan drive motors can indeed be obtained using special transducers (strain gages) placed in contact with the wire at a point prior to its entering each die, which convert the detectable degree of drag into a given output signal. This results in a particularly complex and delicate system, however, and does not ultimately eliminate the risk of wire rupture. The object of the present invention is to overcome the drawbacks mentioned above.